This present invention relates generally to radial-type dynamic pressure fluid bearing systems and, in particular, to foil-type fluid bearing systems comprising a stationary retaining member that surrounds the outer circumference of a rotating journal shaft thereby forming an annular cavity. A foil assembly located in the cavity supports the journal.
Fluid bearing systems are used in many diverse applications requiring high speed rotating machinery. Fluid bearing systems generally comprise two relatively movable elements with a predetermined gap therebetween filled with a fluid, such as air. For example, a fluid bearing system may comprise a stationary bearing housing that surrounds a rotating shaft. Under dynamic conditions, gaps form between the relatively moving surfaces supporting a fluid pressure sufficient to prevent contact between the two relatively movable bearing elements.
Hydrodynamic fluid bearings have been developed by using foils in the gap between the relatively movable bearing elements. The hydrodynamic film forces between adjacent bearing surfaces deflect these foils, which are generally thin, pliable sheets of a compliant material. The foils enhance the hydrodynamic characteristics of the fluid bearing systems and provide improved operation under extreme loads. These foils also function to accommodate eccentricity, runout, and other non-uniformities in the motion of the relatively movable elements. The foils also provide a cushioning and damping effect.
The motion of a rotating element applies viscous drag forces to the fluid in a converging channel. This may result in fluid pressure increases throughout most of the channel. If a rotating element (for example, a shaft) moves toward a non-rotating element (for example, a foil), the fluid pressure increases along the channel. Conversely, if a rotating element moves away, the fluid pressure decreases along the channel.
Consequently, the fluid in the fluid bearing system exerts damping forces on the rotating element that vary with running clearances between the shaft surface and the top foil surface. Higher pressure along the channel provides more fluid film damping forces. These damping forces may stabilize non-synchronous shaft motion and prevent contact between the rotating and non-rotating elements. Any flexing or sliding of the foils may cause coulomb damping which also adds to the radial stability.
Due to preload spring forces or gravity forces, a rotating element of the bearing is typically in contact with the fluid foil members of the bearing at zero or low rotational speeds. This contact may result in bearing wear. Only when the rotor speed is above what is termed the lift-off/touch-down speed will the fluid dynamic forces generated in the channel assure a gap between the rotating and non-rotating elements.
Compliant fluid foil bearing systems typically rely on backing springs and top foils for preload, stiffness, and damping. The foils are preloaded against the relatively movable rotating element to control foil position/nesting and to establish dynamic stability. The bearing starting torque (which should ideally be zero) is proportional to the preload forces. These preload forces also significantly increase the rotational speed at which the hydrodynamic effects in the channel are strong enough to lift the rotating element of the bearing away from the non-rotating members of the bearing. These preload forces and high liftoff/touch-down speeds may result in significant bearing wear each time the rotor is started or stopped.
Conventional foil bearing systems obtain damping from the fluid film between the foil surface and the shaft, and from coulomb friction forces between the foils and undersprings. To increase damping, the typical design increases bearing preload forces that increase both the fluid damping and the coulomb damping. However, this design also increases the contact force between the shaft and foils, resulting in higher start torque before development of the hydrodynamic fluid film.
Conventional foil bearing systems may experience wrapping failure, which may occur when a top foil sticks to a rotating shaft, causing the top foil to undergo tension and tighten around the shaft, in effect, wrapping around the shaft. This wrapping effect dramatically increases the torque required to turn the shaft, which can prohibit turning or damage the bearing by pulling them out of its anchoring mechanism.
One design that attempts to effectively prevent wrapping failure is disclosed in U.S. Pat. No. 5,427,455 to Bosley. A compliant foil hydrodynamic fluid film radial bearing is disclosed, comprising a shaft, a top foil, a spring foil, and a foil-retaining cartridge. The cartridge is located within a bore and has circumferentially undulating cam shaped lobes, or circumferential ramps and joggles, that induce the spring and top foils to form converging fluid-dynamic channels that compress and pressurize the process fluid and diverging channels that draw in makeup fluid. A spring foil is formed as a thin, flat sheet having chemically etched slots of a pattern that cause cantilever beams to stand erect and function as springs when the foil is bent to install in the cartridge.
The Bosley design seeks to lower start torque and stall speed through minimizing radial force transmitted to the shaft. The Bosley design seeks to accomplish this by pushing the top foil circumferentially away from the shaft by using either only a preload bar or a flat circumferential preload spring at the ends of the top foil. Joggles on the top foil are used to ensure fluid film generation.
However, manufacturing difficulties, including costs for additional parts, make the use of preload bars or flat circumferential preload springs costly. Additionally, the level of distributed forces, or preload, between the outer circumference of the shaft and the top foil is very sensitive to the manufacturing variations in the shaft and the bore diameters and the bearing stack-up. Also, the circumferential spring and/or preload bar in the Bosley design and other prior art may keep the top foil from collapsing to the shaft; but the control of radial space between the top foil and the shaft is susceptible to variations in bore diameter and the underspring height. In Bosley's design, if the bore is smaller or if the underspring is taller, the space between the top foil and the shaft will become smaller (and vice versa for short undersprings or larger bore). When the space between the top foil and the shaft becomes too small, too much of the preload from the springs transfers to the shaft through the top foil, dramatically increasing the start torque. If the space between the top foil and the shaft becomes too large, the fluid film damping will decrease dramatically and the rotor will be susceptible to rotor instability.
The prior art is intended for allowing higher preload forces and higher coulomb damping without higher start torque, but does not improve fluid film damping and some suffer from one or more of the following disadvantages:                a) excessive start torque;        b) lower preload forces between the foils and the bore, which may cause lower damping forces;        c) lower tolerances for manufacturing variations;        d) wrapping;        e) higher parts costs.        
As can be seen, there is a need for an improved apparatus for hydrodynamic fluid bearing systems wherein preload forces are transferred from the undersprings to internal circumferential compressive forces within a top foil, resulting in high pre-load between the bore and the top foil, while prohibiting the pre-load to be transferred to the shaft. The top foil should be allowed to expand at high shaft speeds to allow some growth in the film thickness at high shaft speeds, but restricting the film thickness from growing too thick and losing fluid film damping. There is also a need for bearing systems that can accommodate high manufacturing tolerances.